The incorporation of electrically conductive fibers in plastics is well known, as, for example, for reinforcement purposes and/or for improving their electrical and/or thermal conductivity.
However, for some time authorities, as for example in the United States, have been showing concern for environmental hazards of various kinds of electromagnetic radiation, in particular those with high frequencies such as radar waves, microwaves and those produced by signals used in electronic circuits, e.g. in digital devices. The use of radio-frequency and high-frequency electromagnetic radiation will grow in the future as a consequence of the widespread application of microprocessors, digital calculators and weighing scales for cash registers, electronic typewriters, and other personal and business computers with associated peripherals, electronic toys and games, military equipment, etc.
When such devices are housed in metal boxes, they are sufficiently protected against emission of radio-frequency and high-frequency radiation by the metal itself which reflects the emitted radiation towards the box inside. Interference with and disturbance of radio, television or other electronic waves are thus avoided.
However, there is a trend to replace metallic boxes by plastic housings. So far, it has been customary to apply electrically conductive coatings in these plastic housings to provide a shield against the emission of electromagnetic radiation. But a drawback of such coatings is that they are not very durable. Moreover, in most cases, these coatings require special and expensive processing and application methods.
Attempts at imparting electrical conductivity to the plastics themselves (so that they shield against electromagnetic waves) have also been suggested by the incorporation and dispersion of relatively big quantities of conductive fillers. Such conductive fillers include carbon black, aluminum flakes, cut wire, metal coated glass fibers, wire meshes and carbon fibers. However, some drawbacks are associated with these conductive fillers. Some fillers do not permit sufficient dispersion in the plastic matrix and clog together or break excessively and degrade to very small particles so that their shielding effect is strongly reduced. This degradation makes it necessary to add a greater amount of conductive particles which renders a uniform dispersion even more difficult while having a negative impact on the mechanical properties of the material.
Finally, it is known that for effective shielding against electromagnetic radiation the conductive particles in the plastic matrix must possess a considerable aspect ratio, i.e. length-to-diameter (L/D) ratio; these particles must form as much as possible a continuous conductive network in the matrix in order to increase the conductivity without, however, substantially changing the physical and mechanical properties of the plastic matrix.